This research project is designed to study the precise character of synaptic input to neurons in the spinal cord and thalamus of the mammalian central nervous system. Cats and monkeys will be the principal experimental animals, and light and electron microscope methods will be the chief experimental techniques. Specific axonal projections to particular neuronal systems will be identified by causing them to degenerate (Wallerian degeneration) or by labeling the parent cells with radioactive amino acids and studying the axonal projections by autoradiographic methods. Groups of neurons which project from one region of the nervous system to another will be labeled by the retrograde transport of horseradish peroxidase injected into particular sites of the nervous system. The neural systems concerned with somatic sensation will be examined during this project. The details of neuronal interaction elucidated by these studies will be correlated with known physiological properties of these systems, in order to better understand the mechanisms underlying somatic sensation, such as touch and pain.